The present invention generally relates to planar, solid oxide fuel cells and, more particularly, to an apparatus and method of stacking and manifolding unitized solid oxide fuel cells for ready connection and disconnection of gases to manifolds.
A fuel cell is basically a galvanic conversion device that electrochemically reacts a fuel with an oxidant within catalytic confines to generate a direct current. A fuel cell typically includes a cathode material that defines a passageway for the oxidant and an anode material that defines a passageway for the fuel. An electrolyte is sandwiched between and separates the cathode and anode materials. An individual electrochemical cell usually generates a relatively small voltage. Thus, to achieve higher voltages that are useful, the individual electrochemical cells are connected together in series to form a stack. Electrical connection between cells is achieved by the use of an electrical interconnect between the cathode and anode of adjacent cells. Also typically included in the stack are ducts or manifolding to conduct the fuel and oxidant into and out of the stack.
The fuel and oxidant fluids are usually gases and are continuously passed through separate cell passageways. Electrochemical conversion occurs at or near the three-phase boundary of the gas, the electrodes (cathode and anode) and electrolyte. The fuel is electrochemically reacted with the oxidant to produce a DC electrical output. The anode or fuel electrode enhances the rate at which electrochemical reactions occur on the fuel side. The cathode or oxidant electrode functions similarly on the oxidant side.
Specifically, in a solid oxide fuel cell (SOFC), the fuel reacts with oxide ions on the anode to produce electrons and water, the latter of which is removed in the fuel flow stream. The oxygen reacts with the electrons on the cathode surface to form oxide ions that are conducted through the electrolyte to the anode. The electrons flow from the anode through an external circuit and then to the cathode. The circuit is closed internally by the transport of oxide ions through the electrolyte.
In a SOFC, the electrolyte is in a solid form. Typically, the electrolyte is made of a nonmetallic ceramic, such as dense yttria-stabilized zirconia (YSZ) ceramic, that is a nonconductor of electrons which ensures that the electrons must pass through the external circuit to do useful work. As such, the electrolyte isolates the fuel and oxidant gases from one another and allows a potential to build up across it as a result of the difference in electrochemical potential between the fuel and the oxidant. The anode and cathode are generally porous, with the anode oftentimes being made of nickel/YSZ cermet and the cathode oftentimes being made of doped lanthanum manganite. In the solid oxide fuel cell, hydrogen or a hydrocarbon is commonly used as the fuel, while oxygen or air is used as the oxidant.
As mentioned above, the voltage output of a single fuel cell is far too low for many applications. Thus, it frequently becomes necessary to connect multiple fuel cells in series to obtain high voltage power. Additionally, the power demands of many systems require that fuel cells frequently be connected in electrically parallel circuits, thereby providing a greater total current. The physical stacking of multiple fuel cells in series, parallel or series/parallel configuration, however, must incorporate gas-tight connections to allow for a safe and efficient flow of reaction gases. Typically, a group of individual fuel cells are welded, soldered or otherwise bonded together into a single unitary stack, thereby preventing the improper mixing of the reaction gasses, such as in U.S. Pat. No. 5,861,221.
For any given cell, defects can occur during processing. A cell can also become damaged during handling. Because some defects may have been undetected, their negative affects, such as poor performance and consequent effects on its neighboring cells or even the entire stack, are not realized until the cell is placed in the stack. Where adjacent cells are fused or bonded together into a single unitary stack, a single cell that is defectively formed cannot be removed and interchanged with a non-defective cell. At best, the performance of the fuel cell stack becomes impaired. At worst, the entire stack must be discarded due to the failure of a single cell.
In addressing the above drawbacks, the assignee of the present invention has developed a unitized fuel cell that is the subject of U.S. patent application Ser. No. 09/419,343 filed Oct. 15, 1999. The unitized cell includes a first electrically conductive interconnect operatively connected to an anode of the fuel cell. The first interconnect has a first substantially planar portion and a first skirt portion. A second electrically conductive interconnect is operatively connected to a cathode of the fuel cell. The second interconnect has a second substantially planar portion and a second skirt portion, with the second skirt portion being juxtaposed to the first skirt portion. A first salient is formed by a portion of at least one of the first and second skirt portions, with the first salient being disposed at a first edge of the fuel cell. A second salient is formed by a portion of at least one of the first and second skirt portions, with the second salient being disposed at a second edge of the fuel cell. An insulating gasket is disposed between the first and second skirt portions and against the ceramic cell to seal the gases within their respective cell housings. The first and second salients can be attached to a gas manifold by attaching a tube to the skirt of the metal housing. Thus, the fuel cell can be electrically connected with other fuel cells in series and parallel configurations through contacts between metal housings and/or through metal gas manifold tubings. A series connection is made when the anode interconnect of one cell is made in contact with the cathode of its adjacent cell whereas a parallel connection can be made if a metal gas tubing is used to electrically connect similar electrodes of two different cells.
While the use of unitized fuel cells solves many drawbacks in the prior art, design issues relating to the actual stacking and manifolding of fuel cells remain. For example, U.S. Pat. No. 5,298,341 describes prior art as including fuel cell stacks that are arranged in a block configuration. With the stacks positioned adjacent to one another, a manifold is attached to all gas channels of the same orientation. Another prior art design is described as manifolding each stack individually. However, both prior art designs are described as having numerous disadvantages. Thus, U.S. Pat. No. 5,298,341 provides a module having stacks of fuel cells. The fuel cells in each stack are arranged to provide an overall rectangular configuration to the stack. The stacks are oriented on edge and radially spaced apart around a central plenum. The fuel cells in the stacks have gas passageways that extend parallel and perpendicular to the longitudinal axis of the plenum. Circular manifold plates are positioned above and below the module. Each plate has gas flow apertures that coincide with the position of the stacks and a plenum aperture that coincides with the position of the central plenum. In this design, individual stacks may be replaced or repaired but it will be difficult to remove individual cells without affecting the integrity of the neighboring cells.
In U.S. Pat. No. 4,048,385, manifolding is directed to planar, cylindrical shaped fuel cells. The cells include a central active portion surrounded by a frame portion. The frame portions contain duct openings so that when the cells are in a stack, the combined frame portions provide channels extending parallel to the longitudinal axis of the stack. The channels provide inlet and outlet means for different gases. Hollowed out portions in the frame portions allow the passage of gases between the channels and active portions. End plates are then used to sandwich the above components. In this design, holes around the perimeter of the cell can become weak spots that may cause the cell to fracture when placed under the stress of a stack assembly.
Another example of manifolding is in U.S. Pat. No. 4,876,163 that discloses tubular shaped fuel cells with their longitudinal axes aligned parallel to one another. Having such parallel orientation, the fuel cells are arranged in either concentric circles, a spiral, or folded rows. Manifolds are located at the distal ends of the cells. The arrangement was intended to reduce the flow of heat from an interior location of the fuel cell stack to a peripheral location. It was also intended to enable series connection. This design, while being applicable to tubular cells, is not applicable to planar cells.
As can be seen, there is a need for an improved solid oxide fuel cell stack and method of stacking such cells. Another need is for a planar, solid oxide fuel cell stack that provides improved stacking and manifolding. A further need is for a stack design that incorporates unitized fuel cells. Also needed is a fuel cell stack design that minimizes the footprint of the stack. Yet another need is for a fuel cell stack design that allows easy connection and disconnection of gases to the stack.
In one aspect of the present invention, a fuel cell stack comprises a plurality of planar fuel cells juxtaposed to and angularly offset from one another; and a manifold assembly operatively adjacent to the fuel cells, with the manifold assembly being in gas communication with each of the fuel cells.
In another aspect of the present invention, a method of forming a fuel cell stack comprises juxtaposing a plurality of planar fuel cells to one another, with the fuel cells having a pair of planar interconnects that sandwich an anode, cathode, and electrolyte therebetween; orienting the fuel cells in a spiral configuration; interfacing a manifold assembly to the fuel cells; and placing each of the fuel cells in gas communication with the manifold assembly.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.